Substack co-founders Hamish McKenzie (left) and Chris Best (middle)Substack is quietly fundraising on the back of exploding usage of its mobile app and a reignited enthusiasm for new political newsletters, thanks to President Donald Trump.The company believes its mobile application is becoming a powerful force for driving paid subscribers to the 500,000 creators now on its platform.And it doesn’t hurt that Substack is signaling to investors that it could do more to help newsletter writers with s...